1. Field of the Invention
The present invention relates generally to the field of internal combustion engines and, more particularly, to turbocharged internal combustion engines with exhaust gas recirculation (EGR).
2. Description of the Background Art
Turbocharging is a well known method for increasing power output from an internal combustion engine. In a typical turbocharged engine, a turbine uses energy from exhaust gases to power a compressor. This increases the pressure of the air supplied to the engine so that higher cylinder pressure can be achieved thereby improving the performance of the engine.
While improvements in engine performance are desirable, it is also important for an engine to comply with existing emissions requirements. One way in which emissions are reduced to acceptable levels is through the use of exhaust gas recirculation (EGR) wherein a conduit connects the exhaust manifold to the intake manifold to allow exhaust gas to be recycled through the engine. In this manner, exhaust species which are still rich in nitrogen are reintroduced to the engine, lowering NOx emissions levels by lowering flame temperature.
In view of the desire for improved performance and the need for reduced emissions, it is not surprising that turbocharged internal combustion engines with EGR are known in the art. In one engine of this type, exemplified by U.S. Pat. No. 3,925,989 to Pustelnik, a compressor is driven by a turbine connected to the exhaust manifold to pressurize intake air for the engine, and an EGR manifold is connected between the exhaust manifold and the intake manifold to recycle exhaust gases back to the engine to reduce emissions. The EGR manifold includes a one-way valve or a series of valve arrangements to prevent the pressurized intake manifold air from backflowing into the exhaust manifold, and an EGR control system monitors the differential pressure between the intake and exhaust manifolds. When a predetermined pressure differential is established between the intake and exhaust manifolds, the EGR manifold valve opens to recycle exhaust gases.
A disadvantage of turbocharged internal combustion engines with EGR of the above type is that diversion of exhaust gases through the EGR manifold tends to reduce air flow through the compressor. Since compressor map width (i.e., the range of mass air flow over which the compressor is fully functional) decreases with increasing compressor pressure ratio for a given engine speed, any reduction in air flow through the compressor will tend to result in a reduction in surge margin thereby increasing the likelihood of compressor malfunction when there are changes in the air inlet temperature and/or the amount of air flowing through the compressor especially at low engine speeds. Compressor choke problems can also occur at rated load and speed.
In a variation of the above system, exemplified by U.S. Pat. No. 4,215,550 to Dinger et al., a bypass line extends from the intake manifold to a combustion chamber from which it separates into two branches that connect with the exhaust manifold and the EGR manifold, respectively, so that intensely preheated gas from the combustion chamber can be mixed with recycled exhaust gases to overcome unfavorable ignition conditions. While this approach may improve the ability of an engine to start under extreme conditions, the addition of a combustion chamber increases the cost and complexity of the system and may also reduce efficiency and surge margin by significantly increasing pressure ratios.
Thus, there remains a need in the art for improvements in turbocharged internal combustion engines with EGR.
The above-mentioned disadvantages of the prior art are overcome with the present invention, one aspect of which is generally characterized in a turbocharged internal combustion engine assembly with exhaust gas recirculation (EGR) including an air compressor driven by an exhaust turbine, an air intake line for conveying air from the compressor to the engine, an exhaust line for conveying exhaust gas from the engine to the exhaust turbine, an EGR line extending from a first point on the exhaust line downstream of the engine to a second point on the air intake line upstream of the engine, a combustion bypass line extending from a third point on the air intake line upstream of the exhaust gas recirculation line to a fourth point on the exhaust line downstream of the exhaust gas recirculation line, and a pressure adjusting feature disposed along at least one of the air intake line and the exhaust line to maintain the pressure at the fourth point below the pressure at the third point and above the pressure at the second point. Examples of suitable pressure adjusting features include a venturi placed in the air intake line at the second point, a power turbine located along the exhaust line downstream of the exhaust turbine, a split exhaust manifold feeding unequal turbine inlets, and an orifice located along the exhaust line between the EGR line and the bypass line.
Another aspect of the present invention is generally characterized in a method of operating a turbocharged engine assembly with exhaust gas recirculation including the steps of feeding exhaust gas from the engine to an exhaust turbine via an exhaust line, compressing air with an air compressor powered by the exhaust turbine, feeding compressed air from the air compressor to the-engine via-an air intake line, providing an exhaust gas recirculation line between a first point on the exhaust line downstream of the engine and a second point on the air intake line upstream of the engine, providing a combustion bypass line between a third point on the air intake line upstream of the second point and a fourth point on the exhaust line downstream of the first point, and adjusting the pressure in at least one of the air intake line and the exhaust line such that the pressure at the fourth point is below the pressure at the third point and above the pressure at the second point so that a first portion of the exhaust gas from the engine is mixed with a first portion of the compressed air from the compressor for combustion in the engine and a second portion of the compressed air is diverted from the air intake line to the exhaust line without being combusted. In one embodiment, the pressure adjusting step includes the step of passing the first compressed air portion through a venturi so that the second exhaust gas portion is mixed with the first compressed air portion in the venturi. In another embodiment, the pressure adjusting step includes the step of driving a power turbine with exhaust gases from the exhaust turbine via an extension of the exhaust line and connecting the combustion bypass line with the exhaust line extension. In yet another embodiment, the pressure adjusting step includes the steps of feeding exhaust gases from a first set of cylinders to a first turbine inlet and feeding exhaust gases from a second set of cylinders to a second turbine inlet. In still another embodiment, the pressure adjusting step includes the step of passing exhaust gases from the engine through an orifice in the exhaust line such that the pressure downstream of the orifice is lower than the pressure upstream of the orifice.
Some of the advantages of the present invention over the prior art include the ability to operate a turbocharged engine with EGR at lower engine speeds without surge problems and at rated load and speed without compressor choke problems, improvement in air to fuel ratio, simplified construction, and increased power output.